Influence of the Substrate Properties on the Performances of NH3

Nov 23, 2010 - A two-phase 1D+1D dynamic mathematical model of SCR monolithic converters, which explicitly accounts for both gas/solid and intraporous...
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Ind. Eng. Chem. Res. 2011, 50, 299–309

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Influence of the Substrate Properties on the Performances of NH3-SCR Monolithic Catalysts for the Aftertreatment of Diesel Exhaust: An Experimental and Modeling Study Isabella Nova, Djamela Bounechada, Riccardo Maestri, and Enrico Tronconi* Laboratorio di Catalisi e Processi Catalitici, Dipartimento di Energia, Politecnico di Milano, Piazza Leonardo da Vinci 32, 20133 Milano, Italy

Achim K. Heibel, Thomas A. Collins, and Thorsten Boger Corning Incorporated, EnVironmental Technologies DeVelopment, Corning, New York 14831, United States

The effects of structural and geometrical characteristics of wash-coated monolith catalysts on the NONO2/NH3 selective catalytic reduction (SCR) activity were experimentally investigated over the same Cu-exchanged zeolite system coated onto honeycomb cordierite substrates with different cell densities, lengths, washcoat loads, and channel shapes. A stacked configuration was also tested. Contrary to previous reports, it was found that both interphase and intraphase diffusional limitations appreciably affected the deNOx efficiency at intermediate to high temperatures, whereas entrance effects did not play a noticeable role in enhancing the NOx conversion. A two-phase 1D+1D dynamic mathematical model of SCR monolithic converters, which explicitly accounts for both gas/solid and intraporous mass-transfer resistances, successfully predicted all of the observed effects using a single set of rate parameters estimated from intrinsic kinetic runs performed over the same catalyst in powdered form, under diffusion-free conditions. Introduction The catalytic reduction of NOx emission from lean-burn engines is being investigated extensively, with the goal of fulfilling the forthcoming Euro 6 and/or the more stringent U.S. Tier 2 Bin 5 regulations. Currently, NH3- or urea-SCR (selective catalytic reduction) is regarded as the key NOx aftertreatment technology for vehicles running on heavy-duty diesel engines.1 The SCR process for mobile applications can be summarized by the following main reactions: 4NH3 + 4NO + O2 f 4N2 + 6H2O 2NH3 + NO + NO2 f 2N2 + 3H2O

(standard SCR) (1) (fast SCR)

(2) 8NH3 + 6NO2 f 7N2 + 12H2O

(NO2 SCR)

(3)

On commercial V2O5-WO3/TiO2 and metal-promoted zeolite catalysts, the standard SCR reaction (reaction 1) proceeds at temperatures between 250 °C and 450 °C in the presence of excess oxygen:2-5 it is, in principle, the most important SCR reaction, because there is much more NO than NO2 in the engine exhausts. However, the trend is now moving toward improving low-temperature performance through an increase of the NO2/NOx feed content: in fact, a 50% NO2/NOx ratio results in the fastest NOx reduction, associated with reaction 2.1-5 This is the reason why diesel SCR converters are often placed downstream of the diesel oxidation catalyst (DOC): in fact, the main function of the DOC is to oxidize CO and unburned hydrocarbons, but, at * To whom correspondence should be addressed. Tel.: +39 02 2399 3264. Fax: +39 02 2399 3318. E-mail: [email protected].

the same time, it promotes the oxidation of NO to NO2. In such a way, with regard to the downstream SCR catalyst, the importance of the more complex reactions (2) and (3) is increased and an optimum in deNOx efficiency is achieved.4,6 Traditional NH3-SCR catalysts for power-plant applications were based either on extruded bulk monoliths composed of a TiO2 anatase carrier supporting the active components (i.e., V2O5, WO3) or prepared by wash-coating similar V2O5-WO3/ TiO2 catalyst compositions onto cordierite substrates. However, zeolite systems promoted with different transition metals (e.g., Cu, Fe), used primarily in the form of coated honeycomb monoliths,3,5-12 have recently become the most promising class of SCR catalysts. In fact, such new formulations exhibit better high-temperature durability (this is especially important for configurations where the SCR catalyst periodically receives very hot gases from regeneration of the upstream particulate filter) and are less sensitive to nonideal NO2/NOx feed ratios (i.e., different from 1/2, corresponding to the fast SCR reaction (reaction 2)).2,3,5,6 The NH3/urea SCR technology has been already commercialized for heavy-duty diesel vehicles to comply with EURO IV and EURO V regulations.1 Its adaptation to lightduty vehicles and automobiles is also currently under development, as the mass and dimensions of the catalytic converter, as well as the pressure drops, should be further minimized. In this respect, several studies in the literature have reported on the influence of the chemical characteristics of the catalytic washcoat layers in zeolite systems.2,3,6,8,13,14 However, although the role of mass transport in monolithic catalysts recently has been the subject of remarkable theoretical work,15-17 there is a lack of engineering studies addressing experimentally the impact of structural and geometrical characteristics of wash-coated honeycomb catalysts on their global performances. In the specific case of NH3-SCR wash-

10.1021/ie1015409  2011 American Chemical Society Published on Web 11/23/2010

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coated monolith catalysts, the influence of mass-transfer resistances has been typically regarded as negligible in published studies.9,10 However, this conclusion is often based on experimental results collected at relatively low space velocities, which result in total conversions already at intermediate temperatures: thus, such conditions prevent effective establishment of the impact of mass transfer, which, in fact, becomes significant only at temperatures of 300 °C and above. In this work, the main reactions of the NH3-SCR reacting system (i.e., those occurring in the presence of ammonia, NO, and NO2, in different relative amounts) were first systematically investigated over a Cu-promoted zeolite catalyst. At this stage, the catalyst was tested in the form of small particles obtained by crushing the original wash-coated monolith, to eliminate diffusional limitations. Kinetic runs were performed to identify the effects of space velocity, temperature (between 150 °C and 500 °C), pressure (up to 0.8 bar (gauge pressure), NO2/NOx feed ratio (in the full range between 0 and 1), and feed concentrations of oxygen (2%, 8% v/v) and water (5%, 10% v/v). The data were fitted by a global kinetic model, and such intrinsic kinetics were eventually incorporated into a transient heterogeneous 1D+1D mathematical model of NH3-SCR monolithic converters, accounting for both external and internal mass-transfer resistances. Model simulations were then compared with data collected at high space velocities over the same catalyst in the shape of a 400 CPSI (cells per square inch) coated honeycomb monolith, to calibrate the predictions of the masstransfer rates and eventually validate the model. In the second stage of the work, the effects of structural and geometrical characteristics of honeycomb SCR catalysts, including the cell density, the substrate length, the washcoat load, and the shape of the monolith channels, were systematically investigated. The “stacking” effect was also addressed by comparing deNOx activities measured on a single monolith substrate and on two stacked monoliths of equal total length. All the experimental effects were eventually simulated and analyzed using the NH3-SCR converter model. Methods 1. Catalyst Samples. The influence of the substrate properties on the SCR activity was studied over several different samples of the same model SCR catalyst, consisting of cordierite honeycomb monoliths washcoated with the same Cu-exchanged zeolite. The tested samples consisted of small core honeycombs with different cell densities (200, 400, 600 CPSI), lengths (50%-150% of the base length), washcoat loads (60%-155% of the base loading), and cell shapes (square and hexagonal). The SCR catalyst used is representative of typical zeolite-based Cu-technologies wash-coated onto honeycomb substrates for automotive applications. To secure intrinsic SCR kinetic data for reference purposes, NO-NO2/NH3 SCR experiments were also performed in a chemical regime over a packed bed of powdered catalyst, obtained by crushing and sieving (to an average particle diameter of ∼90 µm) a 200 CPSI coated monolith sample. In this case, the powdered catalyst was diluted with an equal amount of cordierite obtained by crushing and sieving a bare substrate, to secure a sufficient depth of the catalytic bed and, thus, avoid bypass effects. 2. Experimental Setup and Procedures. All the SCR runs were performed in a stainless steel (AISI 316) laboratory rig: NH3, NO, NO2, O2, and N2 were fed from calibrated bottled

gas mixtures via mass-flow controllers. Water was dosed by a high-performance liquid chromatography (HPLC) pump into the NOx-O2-N2 stream just upstream of a preheating traced coil (2 m in length), typically maintained at a temperature of 150-200 °C to ensure H2O vaporization. Ammonia (from a bottled NH3-N2 mixture) was eventually mixed into the feed stream immediately upstream of the reactor. The reactor was a stainless steel tube that contained a steel sample holder with a square internal cross section. Typical sizes of the tested monolith catalyst samples were approximately 8 mm × 8 mm × 50 mm. To ensure feed gas pre-mixing, upstream of the catalyst, the reactor was filled with quartz particles (3-5 mm in diameter), followed by a screen containing zirconium balls (1.6 mm in diameter). The reactor was placed in an electric furnace, taking care to position the sample holder in the isothermal region of the furnace. Reaction temperatures were monitored by two thermocouples, located immediately upstream and downstream of the catalyst sample. The gases exiting the reactor reached the analysis section through heated lines (200 °C), to prevent H2O condensation and NH4NO3 deposition.4 Two different gas analyzers were used in a sequential arrangement: an ultraviolet (UV) analyzer (ABB Limas-11HV), for the analysis of NO, NO2, and NH3, was followed by a condenser, for the abatement of NH3 and H2O, and then by a nondispersive infrared (IR) analyzer (ABB Uras-14) for the analysis of N2O. The response time of the analysis system to perturbations of the feed concentrations was ∼5 s. Before each set of experiments, each catalyst sample was conditioned by flowing 2% v/v O2, with the balance being N2, for 1 h at 550 °C. Because of the low concentrations of the reactants, the experiments were not accompanied by significant thermal effects (150 000 h-1) (i.e., much greater than that usually encountered in other literature studies and suitable to prevent total conversion of the reactants already at low temperatures for all NO2/NOx feed ratios): this choice enabled the collection of significant information, particularly on the mass-transfer effects, which, of course, becomes more relevant in the high-temperature region. 3. Kinetic Model. The following nine global reactions were used for the kinetic description of the complete NH3-NO-NO2/ O2 SCR reacting system: NH3 S NH*3

(ammonia adsorption-desorption)

(4)

Ind. Eng. Chem. Res., Vol. 50, No. 1, 2011

4NH*3 + 3O2 f 2N2 + 6H2O

(ammonia oxidation)

(5) 2NO + O2 S 2NO2

4NH*3 + 4NO + O2 f 4N2 + 6H2O (standard SCR reaction) (7) 4NH*3 + 4NO + 3O2 f 4N2O + 6H2O (N2O formation from NO and NH3) (8) 2NH*3 + 2NO2 f NH4NO3 + N2 + H2O (ammonium nitrate formation) (9) 8NH*3 + 6NO2 f 7N2 + 12H2O

(NO2 SCR reaction) (10)

2NH*3 + 2NO2 f N2O + N2 + 3H2O (N2O formation from NO2 and NH3) 2NH*3 + NO + NO2 f 2N2 + 3H2O

(11)

(fast SCR reaction) (12)

The rate equations for reactions 4-7 and reaction 12 were adapted from7,18-21 0 rads ) kads CNH3(1 - ϑNH3)

[

rads ) k°ads exp -

[ (

rox ) k°ox exp -Eox

]

E°des (1 - RϑNH3) ϑNH3 RT

(14)

( )

pO2 1000 1000 ϑNH3 T 473 0.08

)]

1000 C ( 1000 T 473 )](

[

(13)

rNOox ) k°NOox exp -ENOox

√pO

NO

-

2

βO2

(15)

CNO2 KNO2

( ) pO2

βO2

)

[

(

rNO ) k°NO exp -ENO

1000 1000 T 473

)]

(

1 + KNH3

ϑNH3

)

0.08

[

rfast ) k°fast exp -Efast

1000 ϑ ( 1000 T 473 )]

(

(17)

NH3CNO2CNO

( )

PO2 1000 1000 CNOϑNH3 T 473 0.08

)]

[

(

rN2O ) k°N2O exp -EN2O

1000 ϑ ( 1000 T 473 )]

NH3CNO2

(

(21)

CNO2 1000 1000 ϑNH3 T 473 1 + KN2OCNO2

)]

)

(22)

In the case of reaction 11 (i.e., the formation of N2O from NO2 and ammonia), the inhibition effect of NO2 highlighted by the experiments also was included in the rate expression. The intrinsic rate parameters in the above listed equations were estimated by global multiresponse nonlinear regression of transient and steady-state data collected on the powdered catalyst, according to the methods described in ref 7, as well as refs 18-21, based on a simple isothermal isobaric heterogeneous plug-flow model of the packed-bed test microreactor. Because of the large number of adaptive parameters, a sequential fitting strategy was adopted, to minimize correlations between the estimates, as detailed in the following sections. 4. Transport Model. A 1D+1D model that also accounts for intraporous diffusion of the reacting and product species within the porous catalytic washcoat layer was used to simulate the SCR monolithic converter. A detailed description of the SCR reactor model can be found in ref 7 and refs 21-23. Briefly, to represent the external (intrachannel) field, the heterogeneous dynamic model of a single monolith channel includes one-dimensional unsteady differential mass balances of all the relevant gaseous species (NH3, NO, NO2, N2O, H2O, O2; see eq 23) and of the adsorbed species (NH*3 ; see eq 24), as well as continuity equations for all the gaseous species at the gas/solid interface (se eq 25): ∂Ci ν ∂Ci 4 )- kmt,i(Ci - CW i ) ∂t L ∂z dh

(where i ) 1, NCG)

(23) ∂ϑm ) Rm ∂t

(where m ) 1, NCA)

(24)

(where i ) 1, NCG)

(25)

The formation of the unselective product N2O from NO and ammonia (reaction 8) was also taken into account, assuming first-order kinetics in NO and adsorbed ammonia:

[

[

rNO2 ) k°NO2 exp -ENO2

0 ) kmt,i(Ci - CW i ) + Reff,i

(18)

rN2OO ) k°N2OO exp -EN2OO

(20)

Ωm

×

1 - ϑNH3 PO2 βO2

( )

NH3CNO2

(16)

0.08

CNOϑNH3

×

1000 ϑ ( 1000 T 473 )]

[

rAmm ) k°Amm exp -EAmm

(6)

(NO oxidation)

301

βO2

(19)

The oxygen dependence, where relevant, was modeled by adopting a simple power-law approach. For reactions 9-11, which occur when NO2 is present, in addition to NO and ammonia, simple first-order rate expressions in the reactant species concentrations were used:

Symbols are explained in the Notation section. Enthalpy balances for the gas and solid phases also are included, to account for thermal effects:

[

∂Tg ν ∂Tg 4 h(Tg - Ts) )∂t L ∂z dh Fgcp

]

(26)

NCG

∂Ts ) ∂t

h(Tg - Ts) -

∑ ∆H R

i eff,i

j)1

Fscp,sδw[1 + (δw /dh)]

(27)

Notably, the accumulation terms in the gas balance equations (eqs 23 and 26) are customarily neglected, thus introducing a pseudo-steady-state approximation for the gas phase. Nevertheless, such terms were retained in our model, because they granted greater stability of the numerical solution in diagnostic tests. The axial conduction term was not included in eq 27, because a simple analysis (assuming the cordierite thermal

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conductivity to be ∼1 W m-1 K-1) showed this contribution to be smaller than the other terms by 2 orders of magnitude. In eq 23, gas/solid mass transfer and heat-transfer coefficients (kmt,i and h, respectively) were expressed in terms of Sherwood number (Sh) and Nusselt number (Nu), estimated according to a semitheoretical literature correlation,24 as discussed below. Concerning the internal field (washcoat), intraphase diffusional limitations were taken into account by solving, at each axial location, the pseudo-steady-state equations for the diffusion-reaction of the gaseous reactants in the porous catalytic layer, 0 ) Deff,j

∂2C*j ∂x2

+ SW2RjFs

(where j ) 1, NCG)

(28) with the species effective diffusivities evaluated from ε Deff,j ) Dm,j τ

(29)

The local rate of formation of the jth species was NR

Rj )

∑ν r

(30)

jl l

l)1

whereas the effective overall surface rate of formation of j was obtained using the expression Reff,i ) -

Deff,i ∂C*i SW ∂x

|

W

(31)

In eq 29, Deff,i and Dm,i are the effective and molecular diffusivities of species i, respectively; ε is the washcoat porosity; and τ is the tortuosity factor. The model equations were solved numerically according to the method of lines. Backward finite differences and orthogonal collocations with symmetric polynomials were adopted for spatial discretization along the axial and the intraporous coordinate, respectively. The resulting mixed set of algebraic and ordinary differential equations (DAEs) was integrated over time, using the library FORTRAN solver DASPK.25 Results and Discussion 1. Kinetic Runs over a Powdered Catalyst and Kinetic Fit. The rate parameters in eqs 13-22 were estimated by fitting intrinsic kinetic data collected over the powdered catalyst. Such data included a set of both steady-state and transient runs analyzing the different SCR reacting systems (NO/NH3, NO-NO2/ NH3, NO2/NH3) and covering the effects of temperature (150-500 °C), space velocity, oxygen (2% and 8% (v/v)) and water (5% and 10% (v/v)) feed contents, and operating pressure (0, 0.4, and 0.8 bar gauge pressure (barg)). Briefly, the experimental results indicated the following: (i) As expected, NOx conversion was adversely affected by increasing the space velocity for all the investigated NO2/ NOx feed ratios. (ii) Increasing the oxygen feed content3,8,9,19 promoted the activity of the NH3 and NO oxidation reactions (reactions 5 and 6) and that of the NO + NH3 reactions (reactions 7 and 8); such an effect was included in the kinetic scheme by adding a power-law dependence on the local O2 concentration. (iii) No significant effect of water was detected in the investigated range.

All the previously discussed steady-state and transient data were included in the global regression analysis for the estimation of the intrinsic rates of reactions 4-12. Because of the large number of adaptive parameters, a sequential fitting strategy was followed, to minimize correlations among parameter estimates. Thus, analysis of the ammonia adsorption/desorption experiments first secured the estimates of the adsorption/desorption rate constants and of the ammonia storage capacity. The estimates of the rate parameters for the NH3 and NO oxidation reactions (reactions 5 and 6, respectively) then were obtained by regression of the runs involving either NH3 or NO in the presence of different concentrations of oxygen in the feed stream. In a subsequent stage, a set of experimental runs with feed streams containing both NO and ammonia (together with water and oxygen) were analyzed to estimate the rate parameters of reactions 7 and 8. The NO2/NH3 reacting system was then studied to secure the rate constants for reactions 9-11. Finally, the most-complex runs, with feeds including NO, NO2, and NH3, were analyzed to estimate the rate constants of the fast SCR reaction (eq 12). 2. Runs over the Reference 400 CPSI Monolith Catalyst: Mass-Transfer Effects and Model Validation. After completing the fit of the kinetic data over catalyst powder, the intrinsic rate equations were incorporated into the 1D+1D model of the monolith reactor (eqs 23-31), and this was used to simulate base runs over a reference 400 CPSI monolith catalyst sample. At this stage, the purpose was to calibrate the model with respect to external (gas/solid) and internal (intraporous) mass-transfer rates. Gas-solid mass-transfer coefficients in monolith channels were estimated according to the following correlation:24

[ (

Sh ) Sh∞ + 8.827 1000

zL dhScRe

)]

-0.545

[ (

exp -48.2

zL dhScRe

)]

(32)

Equation 32 has a semi-theoretical nature, because it was derived from the fit of theoretical solutions of the Graetz-Nusselt problem of heat transfer to laminar flow in ducts,26 considering also the velocity boundary layer development in the entrance region. It predicts that, already after a short axial distance, the Sherwood number (Sh) approaches an asymptotic value (Sh∞), which is dependent on the honeycomb channel shape.16,24,26,27 SEM photographs of the tested square-channelled honeycomb catalysts showed significant corner rounding effects, because of washcoat deposition: accordingly, an asymptotic value of the Sh number, Sh∞ ) 3.625 (which corresponds to a rounded square channel shape with a r/a ratio of 0.8, where r is the radius of curvature and a is the half side of the square), was selected27 and used in all of the following simulations. However, the sensitivity of the SCR converter model predictions to changing the Sh∞ value from 2.976 (square channel) to 3.656 (circular channel) was determined to be negligible for practical purposes. Gas/solid heat-transfer coefficients were estimated from the same equation (eq 32), after replacing the Schmidt number (Sc) with the Prandtl number (Pr) and the Sherwood number (Sh) with the Nusselt number (Nu). It is worth emphasizing that a heterogeneous 1D model, such as the one herein adopted, requires local gas/solid mass- and heat-transfer coefficients. A popular correlation often used to compute mass-transfer coefficients in honeycomb monoliths is Hawthorn’s correlation,28 which, however, provides lengthaveraged estimates, thus overestimating the local mass transport when incorrectly applied to predict local mass-transfer rates. The distinction between local and length-averaged transfer coefficients is well-known in the heat-transfer literature.26

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Figure 1. Transient run on the 400 CPSI reference monolith catalyst at T ) 200 °C. NH3 step feed (0-500 ppm-0 ppm) in 500 ppm NO, 5% H2O, 8% O2 (N2 balance). Symbols represent experimental outlet concentrations; solid lines represent model predictions.

With regard to internal mass transfer, evaluation of the effective species diffusivities required the calibration of the tortuosity factor in eq 29. After a preliminary sensitivity analysis, the tortuosity factor (τ) was set to a value of 5 in all subsequent simulations. This results in a typical value of ∼3 × 10-6 m2/s for the effective diffusivity of NO at 200 °C. In view of the several uncertainties still affecting the identification of the active sites and their position in the zeolite structure,14 no attempt was made at this stage to also explicitly account for diffusional resistances inside the zeolite crystallites. To validate the model, its predictions were compared with data collected over the reference 400 CPSI monolith catalyst. Figure 1 shows a transient run at 200 °C in the case of the NO/NH3 reacting system; the symbols represent the measured outlet NO, ammonia, and N2O concentrations, whereas the lines are the corresponding model simulations. At ca. t ) 5000 s, ammonia was added stepwise to a NO-containing feed stream; after an initial transient, due to its simultaneous adsorption and reaction, stationary levels for ammonia and NO outlet concentrations were approached, corresponding to NO conversions close to 40%. Notably, some N2O formation (∼30 ppm at steady state) was observed during the experiment. Also, no special transient features were noted when the NH3 feed concentration was restored to 0 ppm (ca. t ) 7400 s). Indeed, it has been reported in the literature that, in similar experiments over vanadium-based and Fezeolite catalysts, as the ammonia was removed from the feed, the NO outlet concentration first decreased, passed through a minimum, and then increased as the adsorbed ammonia species were progressively depleted.3-5,7,10,12,18,29 Such transient behaviors were explained by excess ammonia inhibiting the SCR reaction, according to the existence of an optimal ammonia surface concentration, which is lower than the coverage achieved at steady state. The data in Figure 1 indicate that such an ammonia-inhibiting effect was not appreciable on the tested Cu-zeolite catalyst. Figure 1 shows that the model was able to nicely simulate the performances of the SCR catalyst in the NO/NH3 reactions, also taking into account the transient behavior of the reacting system. Experiments similar to that shown in Figure 1 were run at different temperatures and with different NO2/NOx feed ratios. In Figure 2, the steady-state conversions (symbols) of NOx (Figure 2A) and NH3 (Figure 2B), and the corresponding N2O concentration levels (Figure 2C), measured at different temperatures over the 400 CPSI monolith are compared with model

Figure 2. (A) NOx and (B) NH3 conversions and (C) N2O outlet concentrations measured over the 400 CPSI reference monolith catalyst at different NO2/NOx feed ratios and temperatures. Feed composition: 500 ppm NH3, 500 ppm NOx, 5% H2O, 8% O2 in N2 (balance). Symbols represent experimental results; solid lines represent model predictions.

simulations. Above 300 °C, very high NOx and ammonia conversions (in excess of 75%-80%) were attained for all the investigated NO2/NOx feed ratios. Moreover, it is evident that, in the 300-500 °C range, the catalyst performance was almost independent of the amount of NO2 in the feed. On the other hand, at T ) 200 °C, the NO2/NOx feed ratio was observed to have a strong effect, which determined an increment of the deNOx efficiency upon increasing the NO2 feed content. Also note that the NOx conversions were always lower than the ammonia conversions. In the case of a NO2/NOx feed ratio

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Figure 3. Comparison between (9) experimental NO and (4) NH3 conversions for the NO/NH3 reacting system (500 ppm NH3, 500 ppm NO, 5% H2O, 8% O2 in N2 (balance)) on the 400 CPSI reference monolith catalyst with model predictions obtained using the Tronconi-Forzatti correlation (eq 32, solid lines) and the Votruba correlation (eq 33, dashed lines).

of 1, such behavior was due to the 4:3 NH3/NO2 stoichiometric ratio of the NO2 SCR reaction (eq 10); however, in the case of the standard SCR reaction (reaction 7) and the fast SCR reaction (reaction 12), i.e., NO2/NOx feed ratios of 0 and 0.5, a 1/1 NOx/ NH3 molar consumption would have been expected. Indeed, dedicated experiments showed that, in the high-temperature range, the ammonia oxidation reaction (reaction 5) was active, consuming the reducing agent for the deNOx reactions and limiting the NOx conversions. Consistent with such findings, the results concerning the overall SCR activity will be primarily discussed in terms of NH3 conversion in the following. Figure 2C also shows the N2O outlet concentrations measured in the experiments: an increase of N2O formation upon increasing the NO2 feed concentration was detected, which is consistent with reaction 11. This brings us to the conclusion that higher NO2 feed contents can result in higher conversions, but also in lower N2 selectivities. The solid lines in Figure 2 represent the model simulations: the model seems to have captured the observed trends in the full range of NO2/NOx feed ratios and temperatures, with no systematic deviations. The same conclusion was found for data collected upon changing the other operating conditions (oxygen and water feed concentrations, pressure, and space velocity). To test the sensitivity of the model simulations to external mass transfer, a different correlation was tentatively applied to evaluate the Sherwood numbers. In the past, the Votruba correlation,30

[ ( )]

Sh ) 0.705 Re

4dh L

0.43

Sc0.56

(33)

has been claimed to provide a good description of gas/solid mass transfer in SCR honeycomb catalysts:31 it has an empirical origin that is derived from a fit of experimental data and predicts Sh

values that monotonically decrease along the channel length, with no asymptotic trend. Figure 3 compares simulations of NOx and ammonia conversions obtained using either the Votruba (dashed lines, from eq 33) or the Tronconi-Forzatti (solid lines, from eq 32) correlation with the experimental SCR activity data (symbols) of the NO/ NH3 reacting system collected over the 400 CPSI reference monolith sample. The figure clearly shows that eq 32 leads to predictions that are closer to the experimental results than those based on the Votruba correlation (eq 33). In particular, note that the ammonia conversion profile simulated using eq 33 did not reach 100% at the highest temperatures, contrary to the experimental evidence. This behavior is due to an overestimation of the external mass-transfer limitations related to the unlimited decrease in Sh along the channel axial distance predicted by eq 33, as opposed to the asymptotic limit of Sh predicted by eq 32. Also for different feed compositions and for different substrate characteristics, in fact, the use of the Votruba correlation led to systematic errors in the simulation of hightemperature data. In more-general terms, Figure 3 confirms the significance of external mass-transfer effects in SCR honeycomb monoliths above 300 °C. 3. Effects of Substrate Characteristics. The effects of structural and geometrical characteristics of the honeycomb catalysts (cell density, catalyst length, washcoat load, stacking effect, channel shape) on the NO-NO2/NH3 SCR activity were then investigated by dedicated sets of experiments. In parallel, the SCR monolithic converter model was applied to simulate the same experiments, to assess its predictive quality and quantitatively analyze the effects of the substrate properties. CPSI Effect. The effect of different monolith cell densities was explored by testing three honeycomb samples with 200, 400, and 600 cells per square inch (CPSI), respectively, but with identical overall washcoat loads. The differences in cell density thus resulted in different values of both the channel diameters and the washcoat thicknesses (see Table 1). Figure 4 shows NH3 conversions and N2O concentrations versus temperature measured over the 200 CPSI (denoted by circle symbols), 400 CPSI (denoted by triangle symbols), and 600 CPSI (squares) monolith samples for the three main SCR reacting systems: NO/NH3 (Figure 4A), NO-NO2/NH3 (Figure 4B), and NO2/NH3 (Figure 4C). Data collected over the powdered catalyst (stars), representative of the limiting case of negligible mass-transfer limitations, are also plotted for comparison purposes. In all of the investigated reacting systems, a general positive trend in ammonia conversions was observed, going from the 200 CPSI monolith catalyst to the 600 CPSI monolith catalyst and finally to the powdered sample; in particular, a strong improvement was evident from 200 CPSI to 400 CPSI, while the effect was more limited from 400 CPSI to 600 CPSI. On the other hand, N2O production was not greatly affected by changing the honeycomb cell density. The observed increase in the ammonia conversion with increasing CPSI was evidently associated with the role of mass-

Table 1. Structural and Geometrical Characteristics of the Investigated Honeycomb SCR Catalysts reference catalyst CPSI channel internal diameter washcoat thickness washcoat load length cell shape

400 x x x x square

CPSI effect 200 1.2x 1.2x x x square

600 0.8x 0.7x x x square

washcoat load effect

stacking effect

600 0.8x x/2 0.6x x square

400 x x x x/2 + x/2 square

length effect 400 x x x x/2 square

400 x x x 2x square

cell shape effect 400 0.9x 1.7x 1.5x 0.8x hexagonal

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Figure 5. Effect of internal and external diffusional limitations on ammonia conversion; lines represent model simulations, and symbols represent experimental data collected over (b) the reference 200 CPSI monolith and (f) the powdered monolith in the NO/NH3 reacting system. (Feed: 500 ppm NH3, 500 ppm NO, 5% H2O, 8% O2 in N2 (balance).)

Figure 4. Effect of monolith cell density (CPSI) on NH3 conversion and N2O formation for (A) NO/NH3 reacting system (feed: 500 ppm NH3, 500 ppm NO), (B) NO-NO2/NH3 reacting system (feed: 500 ppm NH3, 250 ppm NO, 250 ppm NO2), and (C) NO2/NH3 reacting system (feed: 500 ppm NH3, 500 ppm NO2). Effects measured over the 200 CPSI monolith (plot a), the 400 CPSI monolith (plot b), the 600 CPSI monolith (plot c), and a powdered monolith (plot d). Symbols represent experimental data; lines represent model predictions.

transfer limitations, whose extent varied, depending on the CPSI of the different samples. Internal mass-transfer limitations were indeed related to the washcoat thickness: higher CPSI values correspond to a thinner washcoat layer and, thus, to lower internal mass-transfer resistances affecting the intermediate temperature region (i.e., between 250 °C and 350 °C).

In the case of external mass transfer, affecting the hightemperature region (i.e., above 350 °C), the key parameter was the channel hydraulic diameter: smaller hydraulic diameters (e.g., for the 600 CPSI) resulted in higher mass-transfer coefficients. Figure 4 also shows that the model simulations (lines) could adequately reproduce the CPSI effect for all the investigated feed compositions. Taking into account that, since the total washcoat load was kept constant, a greater cell density corresponds to a smaller hydraulic diameter, a higher specific surface area of the substrate channels, and a smaller washcoat thickness, the model predicted a reduced contribution of both internal and external mass-transfer limitations upon increasing the monolith cell density, as indeed observed in the experiments. Moreover, as apparent in Figure 4, the same model used for the monolithic samples reproduced the tests on the catalyst powder well when the asymptotic Sherwood number was set to Sh∞ ) 100 and the tortuosity factor was set to a value of τ ) 0.003, to artificially minimize the external and internal mass-transfer resistances, respectively. The model was also able to reproduce the slight differences observed in the selectivity of the different reactions. Generally, N2O production was detected starting from the lowest investigated temperature (150 °C) and in all the reacting systems: as already seen in Figure 2C, in these tests, the amount of N2O formed also was correlated to the NO2 feed content. For a feed consisting of NO2/NOx ) 1, the maximum N2O concentration level of ∼150 ppm was observed at 270 °C. The model predicted the observed results fairly well. Finally, to explicitly identify the individual contributions of internal and external mass-transfer resistances, model predictions of NH3 conversion corresponding to no mass-transfer limitations, internal limitations only, external limitations only, and both internal/external limitations have been plotted in Figure 5 for the case of the NO + NH3 reaction over the 200 CPSI monolith catalyst. The simulations were generated by setting Sh∞ and τ, respectively, to 100/0.003, 100/5, 3/0.003, and 3/5. It is apparent that, according to the model predictions, the deNOx activity of the monolith catalyst was restrained by internal and external mass-transfer limitations to a similar extent at intermediate temperatures, with, however, gas/solid resistances becoming dominant at higher temperatures. Analogous calculations for the case of the 400 CPSI monolith catalyst showed a smaller overall

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Figure 7. Effect of monolith stacking on NH3 conversion and N2O formation for the NO2/NH3 reacting system (feed: 500 ppm NH3, 500 ppm NO2) (lines represent simulations; symbols represent experimental data): single 400 CPSI monolith (plot a) and two stacked 400 CPSI monoliths, each of 50% length (plot b).

Figure 6. Effect of washcoat load on NH3 conversion and N2O formation (lines represent simulations; symbols represent experimental data): (A) NO/ NH3 reacting system (feed: 500 ppm NH3, 500 ppm NO) and (B) NO-NO2/ NH3 reacting system (feed: 500 ppm NH3, 250 ppm NO, 250 ppm NO2). In each panel, plot a denotes data for the 600 CPSI low-coated monolith and plot b denotes data for the 400 CPSI reference monolith.

impact of the diffusional limitations, with, however, still comparable contributions from external and internal resistances. Washcoat Load Effect. Two different catalyst samples were considered to analyze the washcoat load effect, namely, a 400 CPSI monolith sample with reference loading (100% washcoat load) and a 600 CPSI low-coated monolith (60% of the base washcoat load). The corresponding experimental results are plotted in Figure 6 (data points represented by squares and stars, respectively), in terms of NH3 conversions and of N2O formation versus temperature in the case of the NO/NH3 reacting systems (Figure 6A) and the NO-NO2/NH3 reacting systems (Figure 6B). The influence of the different cell densities (400 CPSI vs 600 CPSI) can be regarded as negligible in this context, based on the results shown in Figure 4. As expected, an improvement in the standard SCR deNOx activity was measured in the kinetically controlled lowtemperature region (i.e., below 250 °C), when going from the low-coated sample to the sample with 100% base load (see Figure 6A); however, at higher temperatures, the differences became negligible. This could be explained by the improved mass-transfer characteristics, because of the smaller channel diameter and the reduced washcoat thickness of this sample, in comparison to the reference one. With respect to selectivity (undesired N2O formation), no significant differences were noted.

The effect of the washcoat load was less important when considering the more active fast SCR reaction (Figure 6B), where mass-transfer limitations become significant even at low temperatures. The two tested catalysts showed essentially identical performances, with slight advantages for the 600 CPSI sample at temperatures above 350 °C. Also, the N2O production did not exhibit significant dependence on the amount of the catalytic washcoat, with N2O apparently behaving as an intermediate species. Lines in Figures 6A and 6B represent model simulations and, again, show a good correlation between the experimental and simulated results. Stacking Effect. A “stacked monolith” configuration was tested to explore the impact of enhanced mass-transport rates that are due to the development of velocity and concentration boundary layers in the entrance region of the monolith channels. Two monolith catalyst samples with half the length of the reference samples (25 mm) were prepared for this purpose and loaded in the test reactor in a series configuration with a gap of 3 mm between them. Figure 7 compares the catalytic activities measured over the stacked monoliths with those obtained over the reference single monolith catalyst under identical experimental conditions for the case of the NO2/NH3 reacting system. Concerning the model results, comparative simulations were performed for a single monolith and for the two stacked monoliths: in the latter case, full boundary layer development at the entrance of both monolith pieces was assumed. The model results (solid and dashed lines) in Figure 7 suggest that, in the case of the stacked monolith catalysts, the expected positive effect of the boundary layer redevelopment on gas-solid mass transfer is actually minor and should not be detected. In fact, the experimental results (triangle symbols denote one monolith, square symbols denote two stacked monoliths) confirmed this expectation: no significant effects on both NH3 conversion and N2O formation were indeed observed. The same conclusion also was reached in the case of the other feed compositions (NO2/ NOx ) 0 and 0.5). Thus, the contribution of boundary layer development to the overall mass-transfer rates was negligible in our experiments, likely because of the fact that the boundary layer development is confined in a very narrow initial portion of the monolith length.24,26 Length Effect. To investigate the effect of the monolith length, honeycomb catalysts with 50% and 150% length were tested, in addition to the base sample (100%). In the experiments

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analysis. In the mass-transfer-limited, high-temperature region, the experimental and simulated results were more or less identical for the two catalysts, indicating that the effects of passing from a square to a hexagonal cell shape on the gas/ solid mass transfer rates are indeed minor. The same conclusions were found in the case of the other feed compositions that were tested. The absence of a tangible difference at high temperatures in Figure 8 is explained by the fact that, for both samples, the real cross section of the coated channel has a tendency to be circular, because of the “corner-rounding” effects associated with washcoat deposition: this eventually leads to essentially similar external mass-transfer rates. Conclusions Figure 8. Effect of monolith cell shape on NH3 conversion and N2O formation for the NO/NH3 reacting system (feed is composed of 500 ppm NH3 and 500 ppm NO): square cell (400 CPSI) at reference washcoat loading (plot a) and hexagonal cell at 150% of the base washcoat loading (plot b). Lines represent simulations, whereas symbols represent experimental data.

over the three monolith samples, flow rates were adjusted to maintain the same volumetric gas hourly space velocity (GHSV). It was found that the NOx conversions and the N2O selectivity did not change significantly upon increasing the dimensions of the catalyst samples for all the tested NO2/NOx feed ratios. Hence, the linear velocity inside the monolith channels did not affect the conversion performances, even at high temperatures. This is consistent with the previous results, showing that the Sh value becomes constant after a very short entrance length. The model simulations were also in agreement with these results. Cell Shape Effect. In most commercial applications nowadays, monolithic substrates consist of honeycombs with square channels. Other channel geometries are possible and have been proposed, with some focus on hexagonal channels: indeed, the hexagonal channel shape is associated with a greater asymptotic Sherwood number (Sh∞ ) 3.341) than that of the square channel (Sh∞ ) 2.976), being close to that of the optimal circular shape (Sh∞ ) 3.656).16,26 Therefore, its adoption should, in principle, result in some enhancement of the deNOx activity under conditions limited by external mass transfer. To explore the potential of hexagonal channels, a monolith catalyst sample with hexagonal cell shape was tested and compared with the reference square-celled monolith catalyst. In this case, however, the washcoat load of the hex cell sample was ∼50% greater than in the reference monolith catalyst. Figure 8 shows the experimental results of this comparison in the case of the NO/NH3 reacting system (represented as symbols): the NH3 conversions and N2O concentrations measured with the hex cell sample were somewhat higher in the lowtemperature region (T < 300 °C), whereas, at temperatures of 300 °C and higher, the experimental results were more or less identical. Since this is the temperature regime in which mass transfer becomes relevant, as shown in previous sections, such results demonstrate that the small difference observed at low temperatures is indeed only due to the greater washcoat load of the hex sample. In the simulations (denoted as lines in Figure 8), the differences in both the catalyst mass and the mass-transfer coefficients of the two tested catalysts were considered. The asymptotic Sherwood number for simulation of the reference “square-channelled” monolith catalyst (i.e., Sh∞ ) 3.625) was chosen to account for the real shape of the monolith channel resulting from corner-rounding effects,27 as observed by SEM

Although a large number of recent publications have addressed the catalytic chemistry and mechanisms of the NO-NO2/ NH3 selective catalytic reduction (SCR) reactions, in view of their relevance to the aftertreatment of diesel exhaust, very little has been reported so far on the influence of mass-transfer limitations on the deNOx performance of washcoated monolithic SCR converters. A common but questionable belief is that masstransfer effectssand, particularly, intraporous diffusional resistancesscan be essentially regarded as negligible. For a Cu-promoted zeolite SCR catalyst washcoated onto cordierite monoliths, in this work we have systematically addressed, both by experiment and by simulation, the effects of structural and geometrical characteristics of the substrates, including cell density, cell geometry, catalyst length, stacked configuration, and washcoat load. Our results suggest that mass-transfer limitations do play a role, to an extent which is dependent on the reaction conditions and especially on reaction temperature and NO2 feed content. In fact, the steady-state NOx removal efficiency was found to decrease with decreasing monolith cell density above a threshold temperature, because of both intraphase and interphase diffusional limitations. On the other hand, SCR experiments at the same space velocity over monolith catalysts with different lengths showed no significant length effect on the deNOx activity for all of the NO2/NOx feed ratios that have been tested. The same conclusion applied to the comparison of a single monolith sample with a configuration of two stacked monoliths of identical total length. An effect of the washcoat load was observed only in the absence of NO2 in the feed stream; it leveled off at the typical catalyst loads used today: further increments of the catalyst mass did not result in significant activity enhancements. Additional experiments are needed to determine if the same conclusions also apply under transient conditions. A two-phase 1D+1D dynamic mathematical model of SCR monolithic converters for the aftertreatment of diesel exhaust, which explicitly accounts for both interphase and intraphase diffusional limitations and relies on a semitheoretical correlation for evaluation of the Sherwood numbers, predicted, with good accuracy, all of the effects observed upon changing the substrate characteristics. It is worth emphasizing that the SCR kinetics used in the numerical investigation consisted of a single set of rate equations fitted to data collected in a chemical regime over a powdered catalyst. Integration of such intrinsic kinetics into a 1D+1D monolithic converter model, which includes solution of the diffusion-reaction equations in the washcoat layer, afforded a priori simulation of all the effects of the catalyst structural properties, including the increment of the catalyst load, without any necessity of tuning the rate parameters. This would not be

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possible for more-conventional 1D models of monolithic converters, which empirically incorporate intraporous diffusional effects into “effective” rate parameters. Accordingly, the 1D+1D model seems to be superior when applied to the design and optimization of monolithic substrates for SCR converters. Notation a ) half-side of the square monolith channel [m] cp ) gas specific heat [J kg-1 K-1] cp,s ) solid phase specific heat [J kg-1 K-1] Ci ) gas-phase concentration of species i [mol/mgas3] C*i ) intraporous gas-phase concentration of species i [mol/mgas3] CiW ) concentration of species i at the gas/solid interface [mol/mgas3] Deff,i ) effective intraporous diffusivity of species i [m2/s] dh ) hydraulic diameter of monolith channel [m] Dm,i ) molecular diffusivity of species i [m2/s] h ) gas-solid heat-transfer coefficient [W m-2 K-1] kmt,i ) gas-solid mass-transfer coefficient of species i [m/s] KNO2 ) equilibrium constant for NO oxidation [bar-1/2] L ) monolith length [m] Nu ) Nusselt number NCA ) number of adsorbed species NCG ) number of gaseous species NR ) number of reactions Pr ) Prandtl number PO2 ) oxygen partial pressure [bar] r ) radius of curvature [m] rads ) rate of NH3 adsorption [mol kg-1 s-1] rAmm ) rate of NH4NO3 formation [mol kg-1 s-1] rdes ) rate of NH3 desorption [mol kg-1 s-1] rfast ) rate of fast SCR reaction [mol kg-1 s-1] rN2O ) rate of N2O formation from NO2 and NH3 [mol kg-1 s-1] rN2OO ) rate of N2O formation from NO and NH3 [mol kg-1 s-1] rNO ) rate of standard SCR reaction [mol kg-1 s-1] rNO2 ) rate of NO2-SCR reaction [mol kg-1 s-1] rNOox ) rate of NO oxidation [mol kg-1 s-1] rox ) rate of NH3 oxidation [mol kg-1 s-1] Ri ) local rate of formation of species i [mol kg s-1] Reff,i ) effective surface rate of formation of species i [mol m-2 s-1] Re ) Reynolds number Sc ) Schmidt number Sh ) Sherwood number Sh∞ ) asymptotic Sherwood number Sw ) washcoat thickness [m] t ) time [s] T ) temperature [K] Tg ) gas temperature [K] Ts ) catalyst temperature [K] V ) gas velocity [m/s] x ) dimensionless transverse coordinate (across washcoat) z ) dimensionless axial coordinate Greek Symbols R ) parameter for surface coverage dependence, eq 14 βO2 ) oxygen reaction order δw ) half-thickness of honeycomb wall [m] ∆Hi ) enthalpy of formation of species i [J/mol] ε ) washcoat porosity θj ) surface coverage of species j νi,l ) stoichiometric coefficient of species i in reaction l Fg ) gas density [kg/m3] Fs ) catalyst density [kg/m3]

τ ) tortuosity factor Ωj ) storage capacity of adsorbed species j [mol/kg]

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ReceiVed for reView July 19, 2010 ReVised manuscript receiVed October 19, 2010 Accepted November 3, 2010 IE1015409